Fix for Xpress when stopped due to MAXTIME or MAXNODES

[bknueven]

Fixes # N/A

Summary/Motivation:

If Xpress stops due to some limit, e.g., a time limit set by the user, it leaves the model in a presolved state. Explicitly calling postsolve enables the user to set a time limit or node limit while using Pyomo and get a result back.

Changes proposed in this PR:

• Add postsolve to whenever Xpress calls solve

Questions:

• It would be nice to test this -- are there any existing problems in Pyomo's test suite that might fail on main when a time or node limit is set?

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[codecov[bot]]

Codecov Report

Base: 86.59% // Head: 86.56% // Decreases project coverage by -0.02% :warning:

Coverage data is based on head (4b2ea44) compared to base (3c50ee2). Patch coverage: 80.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2553      +/-   ##
==========================================
- Coverage   86.59%   86.56%   -0.03%     
==========================================
  Files         721      720       -1     
  Lines       80434    80316     -118     
==========================================
- Hits        69652    69527     -125     
- Misses      10782    10789       +7     

Flag	Coverage Δ
linux	83.39% <80.00%> (+<0.01%)	:arrow_up:
osx	73.59% <80.00%> (+<0.01%)	:arrow_up:
other	83.57% <80.00%> (+<0.01%)	:arrow_up:
win	80.60% <80.00%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyomo/solvers/plugins/solvers/xpress_direct.py	73.17% <80.00%> (+0.14%)	:arrow_up:
examples/pyomobook/test_book_examples.py	94.90% <0.00%> (-1.97%)	:arrow_down:
pyomo/solvers/plugins/solvers/GUROBI.py	88.19% <0.00%> (-0.33%)	:arrow_down:
pyomo/contrib/appsi/solvers/gurobi.py	84.36% <0.00%> (-0.12%)	:arrow_down:
examples/pyomobook/performance-ch/wl.py

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[michaelbynum]

@bknueven, I'm sure there are others, but I'm aware of some time limit tests in Appsi (https://github.com/Pyomo/pyomo/blob/064c1203e28f3fc9a6213af9d579fdbc14e03585/pyomo/contrib/appsi/solvers/tests/test_persistent_solvers.py#L821). I think I basically made a random assignment problem large enough to test the time limit.